Capacitance reduction in battery systems

ABSTRACT

A battery system includes a battery cell, a thermally insulating layer, and a thermally conducting layer which includes a fin. The fin pushes against an interior surface of a case which surrounds the battery cell, the thermally insulating layer, and the thermally conducting layer. The thermally conducting layer includes a discontinuity where the discontinuity is configured to reduce a capacitance associated with the thermally conducting layer compared to when the thermally conducting layer does not include the discontinuity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional ApplicationNo. 16/102,315, filed Aug. 13, 2018, and titled “CAPACITANCE REDUCTIONIN BATTERY SYSTEMS,” which is hereby incorporated by reference in itsentirety for all purposes.

BACKGROUND OF THE INVENTION

Some types of batteries are enclosed in a case with a sealed lid. Thisarrangement makes the removal of heat from the inside of the casedifficult. Heat removal is an important aspect of a battery's designbecause some types of (battery) cells can emit large amounts of heatwhen they fail and this heat can cause nearby cells to fail, causingthermal runaway. To address this, new types of battery systems are beingdeveloped which include layers of thermal conductors (e.g., to draw heatout) interspersed amongst the cells. However, because some of thecomponents and/or the arrangement of those components is/are new, thereare unintentional and undesirable side effects. Techniques and/orcomponents to mitigate such unintentional and undesirable side effectswould be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1A is a top view of the stacked contents of a battery sub-module.

FIG. 1B is a perspective view of the stacked contents of a batterysub-module, including a case which compresses the contents of the case.

FIG. 2A is a diagram illustrating an embodiment of the capacitancesacross the fins and tabs of a battery sub-module.

FIG. 2B is a diagram illustrating an embodiment of a total capacitanceproduced by combining smaller capacitances.

FIG. 3A is a diagram illustrating an embodiment of a thermallyconducting layer with circular discontinuities.

FIG. 3B is a diagram illustrating an embodiment of a thermallyconducting layer with rectangular discontinuities on the portion incontact with the battery cell.

FIG. 4 is a flowchart illustrating an embodiment of a process to providea battery sub-module with reduced capacitance.

FIG. 5A is a top view of the stacked contents of a battery sub-modulewhere the interior surface of the case is anodized to create anelectrically insulating surface.

FIG. 5B is a top view of the stacked contents of a battery sub-modulewhere a layer of electrical insulation is placed between the case andthe fins.

FIG. 5C is a top view of the stacked contents of a battery sub-modulewhere the tips of the fins are treated so that they are electricallyinsulating.

FIG. 6 is a diagram illustrating an embodiment of a frame which is usedto hold multiple battery sub-modules.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Various embodiments of a battery sub-module (or, more generally, asystem) with better (e.g., lower) capacitance(s) are described herein.For example, such capacitances could be unintentional, resulting from aspecific combination or arrangement of layers in a battery sub-module.These capacitances are undesirable because they negatively affect theelectrical performance of the battery sub-module (e.g., when a load isbeing powered) and/or may store charge which is then discharged acrosssomeone handling the battery, possibly injuring that person.

In some embodiments, the system (e.g., a battery submodule) includes abattery cell (e.g., a pouch cell), a thermally insulating layer (e.g., alayer of aerogel), and a thermally conducting layer (e.g., to draw heatout and away from other cells) which includes a (e.g., aluminum foil)fin where the fin is configured to push against an interior surface of acase which surrounds the battery cell, the thermally insulating layer,and the thermally conducting layer; the thermally conducting layerincludes a discontinuity where the discontinuity is configured to reducea capacitance associated with the thermally conducting layer compared towhen the thermally conducting layer does not include the discontinuity.As will be described in more detail below, the thermally conductinglayer (e.g., which is designed to draw out heat from a failing batterycell to prevent a nearby battery cell from overheating and failing aswell) unintentionally creates a capacitance and to reduce thiscapacitance, one or more discontinuities (e.g., holes, openings,apertures, cutouts, etc.) are created in the thermally conducting layerto reduce the unintentionally-created capacitance.

First, it may be helpful to show the components described above. Thefollowing figure shows the stacked (e.g., layered) contents of a batterysub-module, including battery cells, thermally insulating layers, andthermally conducting layers.

FIG. 1A is a top view of the stacked contents of a battery sub-module.For clarity and readability, this drawing does not show the case whichsurrounds and compresses the stacked contents. In this example, thestacked content of the battery sub-module includes a repeated patternwhich includes a thermally conducting layer with fins on both sides(100). As shown here, the thermally conducting layer includes threeplanar portions: a left fin, a right fin, and a portion which is incontact with the battery cell (102).

The purpose of the thermally conducting layer (100) is to act as a heatsink for the battery cell (102) which is in contact with that thermallyconducting layer. By removing the heat produced by the battery cell(e.g., during normal operation and/or a catastrophic failure) from theinterior of the stacked layers to the exterior, this prevents nearbybattery cells from overheating and possibly failing.

Structurally, the fins are bendable and act like a spring and push backwhen pressure is applied. This enables the thermally conducting layer(e.g., via the fins) to make contact with the interior surface of thecase (not shown) even if there is some (e.g., air) gap around the finand/or variance in this distance. For example, even if the edges of thelayers are not perfectly aligned and/or the layers have differentwidths, the thermally conducting layer is still able to make contactwith the interior surface of the case. The thermally conducting layer isbetter able to conduct heat when the fin is in contact with the walls ofthe case (e.g., possible via some intervening material or substance), sohaving the fins act like a spring ensures that the fins always touch thecase and are better able to bring heat out from the interior of thestacked contents shown. In some embodiments, the thermally conductinglayer is made of metal (e.g., 1235 series A1) because metal is a goodthermal conductor and also permits the fin to act like a spring.

The next layer in this example pattern is a battery cell (102). In thisexample, the battery cells are pouch cells. Pouch cells perform betterwhen pressure is applied (e.g., ˜3-5 PSI). More specifically, the cyclelife of pouch cells can be extended by applying pressure to the pouchcells. For this reason, the stacked layers shown here are compressedusing a metal case (not shown).

The next layer is a thermally insulating layer (104) and is sometimesreferred to herein more simply as insulation. In this example, becausethe insulation (like all of the layers shown here) will be compressed,the insulation is made up of a material which can withstand (e.g.,without collapsing) the expected pressure from the compressed can. Forexample, using the spring constant of a material as a metric ofinterest, the spring constant of the insulation should benon-negligible. In some embodiments, the insulation is made of aerogelwhich is a good thermal insulator and has a non-negligible springconstant.

Thermally, the layers of insulation prevent (or at least slow downand/or mitigate) heat from spreading from one cell to another cell. Forexample, suppose one cell fails catastrophically and in the processreleases a large amount of heat. Without any insulation, all of thatheat would propagate to nearby cells and cause those cells to also failcatastrophically. Eventually, all of the cells would failcatastrophically in a domino-like effect. This positive feedback cycle,domino-like effect (e.g., at the cell or battery level) is sometimesreferred to as thermal runaway. The layers of insulation prevent (or atleast slow down and/or mitigate) thermal runaway from happening (atleast at the cell level).

This layering pattern repeats. In some embodiments, each batterysub-module includes 12 battery cells and a corresponding number ofthermally insulating layers and thermally conducting layers. No specificbeginning and ending to the stacking pattern is shown here and anyappropriate beginning and ending layer(s) may be used. In someembodiments, the stacked layers begin and end with two layers ofinsulation.

The following figure shows a perspective view of the stacked contents ofthe battery sub-module, this time with the case.

FIG. 1B is a perspective view of the stacked contents of a batterysub-module, including a case which compresses the contents of the case.From this view, the positive tab (106) and negative tab (108) can beseen extending upwards out of the case (150). The positive tabs andnegative tabs are respectively connected to each other electrically (notshown) so that when the contents of the case are sealed with a lid, thelid exposes a single positive connection or port and a single negativeconnection or port.

Returning to FIG. 1A, earlier prototypes of the battery sub-modulerevealed that there was an unintended and/or unexpected capacitancebetween the thermally conducting layer (100) and the positive tab (106)and/or negative tab (108). This is due to the material used to make thethermally conducting layer (i.e., aluminum) in the prototypes. Aluminumis electrically conducting and so charge can build up on the thermallyand electrically conducting layer (100). For example, each battery cell(102) has a positive tab (106), referred to more generally as a positiveconnector, and a negative tab (108), referred to more generally as anegative connector. Suppose that the (unintended) capacitance across asingle tab and fin pair (e.g., positive tab (106) and right fin (110))is 3 nF. Collectively, these individual capacitances add up to anon-negligible value. The following figures show an example of this.

FIG. 2A is a diagram illustrating an embodiment of the capacitancesacross the fins and tabs of a battery sub-module. In the example shown,capacitors 200, 202, and 204 represent the individual capacitancesacross each fin-tab pair (a shorter and more convenient name to refer tothe capacitance across the tab of a battery cell and the fin of athermally conducting layer) corresponding to the arrangement shown inFIG. 1A where there are n pairs of fins and tabs. For example,C_(fin-tab,1) (200) is the capacitance across a first fin-tab pair,C_(fin-tab,2) (202) is the capacitance across a second fin-tab pair, andso on where the capacitances are connected together in parallel. In oneexample, there are 12 fin-tab pairs in each battery sub-module and sothere would be 12 capacitances connected together in parallel.

Electrically, these individual capacitances combine (e.g., additively)to create a large total or overall capacitance. The following figureshows an example of this.

FIG. 2B is a diagram illustrating an embodiment of a total capacitanceproduced by combining smaller capacitances. In the example shown, thesingle total capacitance (250) shown here corresponds electrically tothe circuit shown in FIG. 2A. If there are 12 fin-tab pairs and eachindividual capacitance across a single fin-tab pair is ˜3 nF then thetotal capacitance (250) is ˜36 nF. In at least some applications, thisis a non-negligible total capacitance.

In one example application, multiple battery sub-modules are (further)combined together into larger battery units or systems and are used topower an all-electric vehicle, such as an aircraft. This is how theearlier prototypes were used. In the example aircraft application, thelarger battery units or systems (e.g., produced by combining multiplebattery sub-modules) supplied voltages on the order of 600V (e.g.,because the propulsion system of the aircraft requires such a highvoltage). This combination of high voltages and non-negligible (albeitunintentional) capacitances can be dangerous for workers who handle thebatteries because they could be seriously hurt or even killed ifhigh-voltage charge builds up on these unintentional capacitances andthen discharges through the worker. The unintentional capacitances alsoaffect the electrical performance of the system. For this reason, it isdesirable to reduce the unintentional capacitances created by thethermally conducting layer.

The following figures show some examples of a thermally conducting layerwith one or more discontinuities to reduce an unintentional capacitanceproduced by that thermally conducting layer.

FIG. 3A is a diagram illustrating an embodiment of a thermallyconducting layer with circular discontinuities. In the example shown,the thermally conducting layer has three planar portions: a left fin(300), a portion which is in contact with the battery cell (302), and aright fin (304). In this particular example, the part of the thermallyconducting layer that is flush with the battery cell (302) has aplurality of circular discontinuities (306). Capacitance is reduced byreducing the surface area of the part of the fin that is in contact withthe cell surface (302). Discontinuities in the left fin (300) and rightfin (304) do not meaningfully change the capacitance of the tabs to thefin and therefore are not shown in this example. It is noted that theshapes, sizes, numbers, and/or placements of the discontinuitiesdescribed herein are merely exemplary are not intended to be limiting.

The discontinuities (306) reduce a capacitance associated with thethermally conducting layer compared to when the thermally conductinglayer does not include the discontinuity. The discontinuities do this byreducing the surface area of the thermally conducting layer in closeproximity to the cell which in turn reduces the capacitance produced bythe exemplary thermally conducting layer (e.g., between the thermallyconducting layer and a battery cell). For example, this means that eachof the individual capacitances (C_(fin-tab,i)) shown in FIG. 2A havesmaller values and correspondingly the total capacitance (C_(total))shown in FIG. 2B is also smaller. The number and/or placement of thediscontinuities may be adjusted to adjust the capacitance as desired.Generally speaking, the more the discontinuities reduce the surface areaof the thermally conducting layer, the more the capacitance willdecrease.

Thermally, the discontinuities do not significantly affect the abilityof the thermally conducting layer shown to draw heat out of the centerof the stacked layers to the edges of the stacked layers andsubsequently out via the case (not shown). Naturally, the shape, number,and/or placement of the discontinuities may be adjusted to find anacceptable tradeoff between thermal conductivity and reducedcapacitance. As an example, the following figure shows anotherembodiment of a thermally conducting layer.

FIG. 3B is a diagram illustrating an embodiment of a thermallyconducting layer with rectangular discontinuities on the portion incontact with the battery cell. In this example, the planar part of thethermally conducting layer that is flush with the battery cell (310) hasrectangular discontinuities (312) which extend from near the shared edgewith the left fin towards the shared edge with the right fin. In someembodiments, this is attractive because it creates (e.g., horizontal)thermal channels or corridors via which the heat can be drawn out of theinterior of the stacked layers towards the fins. Another way ofdescribing this shape and layout is to say that the discontinuities onthat surface have a first distal end pointing towards the left fin and asecond distal end pointing towards the right fin.

Although vertical thermal channels which extend towards the top edge andbottom edge could be created using discontinuities, this may not be aseffective as the horizontal thermal channels shown here because the topedge and bottom edge do not have fins which guarantee contact with theinterior surface of the case (not shown). The spring-like nature of theleft fin and right fin ensures that the fins make contact with the case,even if there is some variation in the space or gap between the case andthe left edge and/or fin and right edge and/or fin.

The following figures show a process for providing a battery sub-modulewith improved (e.g., reduced) capacitance.

FIG. 4 is a flowchart illustrating an embodiment of a process to providea battery sub-module with reduced capacitance. In some embodiments, theprocess is performed by and/or using the devices shown in FIGS. 1A-1Band 3A-3B.

At 400, a battery cell is provided. See, for example, the battery cells102 in FIG. 1A.

At 402, a thermally insulating layer is provided. See, for example, thelayers of thermal insulation (104) in FIG. 1A. As described above, theselayers of insulation prevent (or at least mitigate) the transfer of heatfrom one battery cell to another battery cell. As described above, afailing battery cell may produce large amounts of heat, and the layersof thermal insulation prevent (or at least mitigate) other battery cellsfrom overheating which could cause those batteries to fail and which inturn could cause still more battery cells to fail, resulting in acascading failure scenario (i.e., thermal runaway).

At 404, a thermally conducting layer which includes a fin is provided,wherein: the fin is configured to push against an interior surface of acase which surrounds the battery cell, the thermally insulating layer,and the thermally conducting layer; and the thermally conducting layerincludes a discontinuity, wherein the discontinuity is configured toreduce a capacitance associated with the thermally conducting layercompared to when the thermally conducting layer does not include thediscontinuity. See, for example, FIG. 1A which shows a top view of thestacked layers, including the fins (100) which push against the case(see case 150 in FIG. 1B). See also discontinuities 306 in FIG. 3A anddiscontinuities 312 in FIG. 3B.

In some embodiments, other features and/or techniques are used to reducecapacitance (e.g., at any scope or level, such as at the thermallyconducting layer, at the sub-module level, at the battery system ormodule level, etc.). The following figures show some examples of othercapacitance reduction techniques and/or features. Naturally, thesetechniques and/or features may be used in any combination, includingcombinations which are not specifically described herein.

FIG. 5A is a top view of the stacked contents of a battery sub-modulewhere the interior surface of the case is anodized to create anelectrically insulating surface. In this example, the interior surface(500) of the case has been anodized. In this example, the anodizationprocess has only been applied to the inside of the case so theexterior-facing part of the case (502) is not anodized. In otherembodiments, the entire aluminum can may be anodized. The case is madeof aluminum in this example and anodizing aluminum increases theelectrical insulation. As such, the interior surface has betterelectrical insulation than before anodization occurred.

The electrically insulating interior surface of the case increases theelectrical insulation between fins (e.g., a first fin on the left sideand second fin on the left side). This has the effect of “breaking up”the parallel arrangement of capacitors shown in FIG. 2A so that they areno longer in a parallel arrangement, at least electrically. As a resultof the new, non-parallel electrical arrangement, the correspondingcombined or total capacitance is reduced. For example, instead ofproducing a total capacitance of 12×3 pF=36 pF, it is a smaller value.As described above, reducing the capacitance is desirable.

It is noted that the battery sub-module still needs to bring heat outfrom inside the case (e.g., from the stacked layers in the core of thebattery sub-module). As such, the anodized interior surface (or anyother electrical insulation serving this purpose) should not interferesubstantially with the removal of heat from the battery sub-module. Forexample, a thermal conductivity decrease on the order of 20% (e.g., inthe range of 18%-22%) would be acceptable.

In this example, the electrical insulation is achieved by anodizing theinterior surface of the case. However, the electrical insulation can beadded using a variety of techniques. The following figures show someother examples.

FIG. 5B is a top view of the stacked contents of a battery sub-modulewhere a layer of electrical insulation is placed between the case andthe fins. In this example, a layer of electrical insulation (510) isplaced between the case (512) and the fins (514). This may be performedin a number of ways. If the electrical insulation is “floppy” and cannotstand on its own (e.g., it is not sufficiently stiff), the stackedcontents may be first bundled together and then wrapped with the layerof electrical insulation. The wrapped contents with the “wrapper” ofinsulation can then be inserted into the can.

Alternatively, if the electrical insulation can stand on its own, thelayer of insulation may be placed into the case first, and then thestacked layers are inserted into the case. These are some examples andare not intended to be limiting. As before, the material used for theelectrical insulation (510) is selected so that it does notsignificantly interfere with the removal of heat.

FIG. 5C is a top view of the stacked contents of a battery sub-modulewhere the tips of the fins are treated so that they are electricallyinsulating. In this example, instead of electrically insulating theinterior surface of the case (552), the tips of the fins (554) aretreated, coated, or otherwise processed so that they have a layer ofelectrical insulation (556). In some embodiments, an electricallyinsulating compound, (e.g., thermal) grease (e.g., which includespolymeric matrix and a metal-oxide filler), resin, or adhesive isdeposited on the tips of the fins. Alternatively, the fins (especiallythe tips) are anodized. As before, the deposited material or processingtechnique used does not substantially increase the thermal insulation.

A more general way of describing the embodiments described above is tosay that a path (e.g., electrical and/or thermal) which includes one ofthe fins and the case includes an insulator that increases electricalinsulation and does not substantially increase thermal insulation. Asshown above, this (e.g., electrical but not thermal) insulation may beinserted or otherwise implemented using a variety of techniques,including by being formed by anodizing the interior surface of the case,a layer of insulating material between fin and the case, a layer ofinsulating material deposited on at least a tip of the fin, being formedby anodizing at least a tip of the fin, etc.

In some embodiments, battery assembly techniques may dictate which isthe more attractive embodiment. For example, if the assembly techniquedoes not include compressing the fins while the stacked layers areinserted into the case, then the embodiments shown in FIG. 5A or FIG. 5Cmay be more attractive. This is because the separate layer of electricalinsulation between the case and the fins may get caught on the caseand/or fins when they are being pushed together. However, if the finsare compressed during insertion, then this may not be a problem. To putthis more generally, a variety of considerations (e.g., cost, batteryassembly technique, the tradeoff between improved/increased electricalinsulation and degraded/decreased thermal insulation for a givenmaterial or processing technique, etc.) may be considered in selectingthe appropriate embodiment.

As described above, in some embodiments, multiple battery sub-modulesare combined together to form a larger, high-voltage battery system. Thefollowing figure shows an example of this and how an associatedcapacitance can be reduced.

FIG. 6 is a diagram illustrating an embodiment of a frame which is usedto hold multiple battery sub-modules. As described above, in one exampleapplication, multiple battery sub-modules (e.g., each of which outputs avoltage on the order of 15V) are combined together to generate a highvoltage supply (e.g., on the order of 600V) to power high-voltage loads,such as lift fans. To do this, the exemplary frame (600) is used. Theframe has two accessible sides into which battery sub-modules (602) areinserted and held in a detachably coupled manner: the side shown hereand the opposite side.

To secure each of the battery sub-modules to the frame, two screws (604)are inserted through holes in the case (606): one at the top of the caseand one at the bottom of the case. The screws then go through holes inthe frame (608) to secure the battery sub-module to the frame.

In the example shown, the battery sub-modules have their lids on. Thelids have two output connectors or ports for the power supply (610), onepositive and the other negative. These power supply connectors areconnected together using cables and/or electronics which are not shownin this diagram so as not to obstruct the components shown. For example,these cables and/or electronics are coupled to the lids and/or face theopen side shown here.

In this example, because the frame is made of an electrically conductivematerial, as are the cases, this leads to the undesirable addition ofparallel sub-module capacitances, similar to how a conductive sub-modulecase can lead to the undesirable addition of parallel cell-to-fincapacitances (see FIG. 2A and FIG. 2B). With the battery sub-modulesscrewed into the frame, the combine together in an undesirable manner.Similar to the smaller-scale examples described above, this creates acapacitance which can shock workers and/or interfere with the electricalperformance of the system.

To reduce the capacitance, in some embodiments, the frame haselectrically independent shear panel rails (612) which better isolate(e.g., electrically) the battery sub-modules from each other. This actsto prevent the capacitance of each rail from adding together (or leastcombines them in a manner that produces a smaller value).

A more general way to describe this is to say that a frame which isconfigured to hold a first battery sub-module and a second batterysub-module includes an insulating portion that is configured to increaseelectrical insulation between the first battery sub-module and thesecond battery sub-module. In this example, this is done by making theshear panels (which make contact with the cases of the batterysub-modules) with electrically insulating and/or non-conductivematerial. In some embodiments, the shear panel is coated or processed toproduce the electrical insulation.

Unlike the previous examples, heat transfer is not as much of a concernhere and so the insulation between battery sub-modules can affectthermal conductivity. This is because the frame is not the primarythermal pathway via which heat from the battery sub-modules isdissipated. In contrast, with some of the examples of above, primarythermal pathways could be affected and so material and/or techniques areselected accordingly (i.e., to not substantially impede the removal ofheat from the interior of the battery sub-modules).

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system, comprising: a battery cell; a thermallyinsulating layer; and a thermally conducting layer which includes a fin,wherein: the fin is configured to push against an interior surface of acase which surrounds the battery cell, the thermally insulating layer,and the thermally conducting layer; and the thermally conducting layerincludes a discontinuity, wherein the discontinuity is configured toreduce a capacitance associated with the thermally conducting layercompared to when the thermally conducting layer does not include thediscontinuity.